Revert Breaking Change: v1.9.1 introduced a breaking change where model fields were deep copied by default, this release reverts the default behaviour to match v1.9.0 and before, while also allow deep-copy behaviour via copy_on_model_validation = 'deep'. See #4092 for more information.

• Allow for shallow copies of model fields, Config.copy_on_model_validation is now a str which must be 'none', 'deep', or 'shallow' corresponding to not copying, deep copy & shallow copy; default 'shallow', #4093 by @timkpaine